:| u came on wrong time , on the release of 1.3.1 it doesnt support alot of mods but ill tell u all i can
first of go to the files where you installed minecraft to do this you must
1-start run
2-type %appdata%
3-open folder called .minecraft
so in that place i prefer u make a shourtcut for it by copying the link , go to desktop and create new shortcut and post that link in it -it will make life easier-
anyways if u wanna install a texture pack
1-go to the .minecraft then go to texutre pack folder and paste the texture pack you downloaded... here are some texture paacks links http://www.minecraftdl.com/category/minecraft-texture-packs/
to install mods ,
1-go to .minecraft
2-choose bin folder
3-open minecraft.jar with win rar
4-MOST IMPORTANT PART REMOVE META-INF,it prevent from loading mods
5-paste any mod u want over there + make sure you back up the jar
hmm i guess thats all u need to know.oh wait
some mods may require modloader or forge mod, these 2 mods will give u ability to put the mods in sepret folder "not in the jar but they must be installed in the jar"
REMEMBER REMOVE META-INF